A high electron mobility transistor (HEMT) is a commonly used device in wireless communications, such as low noise amplifiers for RF (radio frequency) signals and switch elements in integrated wireless circuits.
FIG. 1 is a cross section view for a conventional HEMT device, which comprises sequentially: a substrate 101, a channel layer 103, a spacing layer 105, a δ doped layer 107, a Schottky layer 109, a first etch stop layer 111, a first n type doped layer 113, a second etch stop layer 115, and a second n type doped layer 117. The channel layer 103 is formed epitaxially on the substrate 101. The spacing layer 105 is formed above the channel layer 103. The δ-doped layer 107 is formed above the spacing layer 105. The Schottky layer 109 is formed above the δ doped layer 107. The first etch stop layer 111 is formed above the Schottky layer 109. The first n type doped layer 113 is formed above the first etch stop layer 111, which is an n-GaAs layer. The second etch stop layer 115 is formed above the first n type doped layer 113. The second n type doped layer 117 is formed above the second etch stop layer 115, which is an n+GaAs layer. The gate recess 135 is formed and ended at the Schottky layer 109 by etching, and then a metal layer acting as the gate electrode 121 is deposited inside the gate recess 135 to form Schottky contact to the Schottky layer 109. A metal layer acting as the source electrode 123 is deposited on one end of the second n type doped layer 117 to form ohmic contacts. A metal layer acting as the drain electrode 125 is deposited on one end of the second n type doped layer 117 to form ohmic contacts. This kind of structure has been widely used in the past. The advantage of this structure is that a superior Schottky contact between the gate electrode 121 and the Schottky layer 109 can be obtained since the Schottky layer 109 is made of a medium energy gap material. However, this structure also has a drawback. When the device is at on state, it will have a large on-state resistance, Ron, due to the large difference in energy gap between the Schottky layer 109 and the second n type doped layer 117 and the first n type doped layer 113.
To overcome the drawback of large Ron, an improved HEMT structure had been developed, which is shown in FIG. 2. FIG. 2 is a cross section view for another conventional HEMT device, which comprises sequentially: a substrate 201, a buffer layer 202, a channel layer 203, a spacing layer 204, a δ-doped layer 205, a Schottky layer 206, an undoped layer 207, a etch stop layer 208, a first n type doped layer 209. The buffer layer 202 is formed above substrate 201. The channel layer 203 is formed above the buffer layer 202. The spacing layer 204 is formed above the channel layer 203. The δ-doped layer 205 is formed above the spacing layer 204. The Schottky layer 206 is formed above the δ-doped layer 205. The undoped layer 207 is formed above the Schottky layer 206, which can be formed of an undoped i-GaAs layer, an undoped i-In0.5Al0.5As layer, or an undoped i-In0.5Ga0.5As layer. The etch stop layer 208 is formed above the undoped layer 207. The n type doped layer 209 is formed on the etch stop layer 208, which is an n+GaAs layer. By etching, a gate recess is formed and ended at the Schottky layer 206, a source recess is formed and ended at the δ-doped layer 205, and a drain recess is formed and ended at the δ-doped layer 205. Then the gate electrode 217 is deposited inside the gate recess to form Schottky contacts to the Schottky layer 206. A metal layer acting as the source electrode 220 is deposited inside the source recess to form ohmic contacts to the δ-doped layer 205. A metal layer acting as the drain electrode 221 is deposited in the drain recess to form ohmic contacts to the δ-doped layer 205. The source electrode 220 and the drain electrode 221 can form direct contact to the n type doped layer 209, the undoped layer 207, the Schottky layer 206, and the δ-doped layer 205 through the source recess and the drain recess, such that the on-state resistance, Ron, of the device can be reduced. However, because of the material used in the undoped layer 207, and contacts to the δ-doped layer 205, the reduction of the on-state resistance Ron is not good enough, and the application with the devices is limited.
In view of these facts and for overcoming the drawback stated above, the present invention provides an improved HEMT structure and a fabrication method thereof The devices according to the present invention not only have a low resistance Ron at on state, but also enhance the DC-RF performance of the device. Furthermore, the fabrication process for the devices has a high stability and the fabricated devices have good reliability.